Warm Dense Matter
Warm dense matter (WDM) is the stuff believed to be at the cores of giant gas planets in our solar system and some of the newly observed “exoplanets” that orbit distant suns, which can be many times more massive than Jupiter. Their otherworldly properties, which stretch our understanding of planetary formation, have excited new interest in studies of this exotic state of matter. WDM remains largely mysterious because it is difficult to create and study in a laboratory, can exhibit properties of several types of matter and occupies a middle ground between solid and plasma. Our own sun is an example of a self-sustaining plasma, and plasmas have also been harnessed in some TV displays.
While warm dense matter is believed to exist in a stable state at the heart of giant planets, in a laboratory it lasts just billionths of a second. Scientists have relied largely on computer simulations, driven by scientific theories, to help explain how a solid, when shocked with powerful lasers, transforms into a plasma.
A team led by SLAC High Density Energy Science division director Siegfried Glenzer used laser light to compress ultrathin aluminum foil samples to a pressure more than 4,500 times higher than the deepest ocean depths and superheat it to 20,000 kelvins – about four times hotter than the surface of the sun. SLAC’s Linac Coherent Light Source (LCLS) X-ray laser, a DOE Office of Science User Facility, then precisely measured the foil’s properties as it transformed into warm dense matter and then into a plasma – a very hot gas of electrons and supercharged atoms. LCLS, with its complement of high-power lasers, is uniquely suited to creating and studying matter at the extremes. Its ultrabright X-ray pulses are measured in femtoseconds, or quadrillionths of a second, so it works like an ultra-high-speed X-ray camera to illuminate and record the properties of the most fleeting phenomena in atomic-scale detail.
In this experiment, researchers used a high-power optical laser at LCLS's Matter in Extreme Conditions (MEC) experimental station to fire separate beams of green laser light simultaneously at both sides of coated, ultrathin aluminum foil samples, each just half the width of an average human hair. The lasers produced shock waves in the material that converged to create extreme temperatures and pressures. They struck the samples with X-rays just nanoseconds later, and varied the arrival time of the X-rays to essentially make a series of snapshots of warm dense matter formation. The team used a technique known as small angle X-ray scattering to measure the internal structure of the material, capturing its brief transition into the warm dense state.
The results of the SLAC study, published March 23 in Nature Photonics, could also lead to a greater understanding of how to produce and control nuclear fusion, which scientists hope to harness as a new source of energy.
Studying WDM Using Ultrafast Electron Diffraction (UED) Techniques
Structural dynamics plays a significant role in WDM science and is yet to be fully understood, especially under non-equilibrium electron and ion temperature conditions created by ultrafast optical excitation. Experimentally, measuring the structural changes during solid/solid and solid/liquid phase transitions are particularly challenging due to their rapid evolution and the disorder of the ensuing structure. One technique to tackle these challenges is known as ultrafast electron diffraction (UED). As compared to X-rays, electrons have much larger (104 –106 times) scattering cross sections and their elastic mean-free-path match better with optical pump excitation depth, making them the ideal choice to study WDM created by optical excitation of nanometer thick samples.
The experiments of UED-WDM were performed with the MeV-UED beam line located at SLAC’s Accelerator Structure Test Area (ASTA) facility. By using relativistic electrons, the inherent space charge effect, which tends to broaden the bunch, is significantly suppressed. Therefore high bunch charge and femtosecond temporal resolution become feasible for this technique. We have recently carried out thorough experimental studies on the melting dynamics of ultrafast laser pulse excited warm dense gold and radiation damaged tungsten using this facility. An example of our measurements on single crystal gold is shown in the movie.
The above movie shows the temporal evolution of the electron scattering pattern of a 35-nm-thick freestanding single-crystal gold foil pumped by 400 nm, 130 fs (FWHM) laser pulse with pump fluence of 180 mJ/cm2. Each frame was obtained with a single shot measurement, demonstrating the advantage of high SNR for this MeV-UED device. As indicated, one can see the Bragg peaks decrease with time in intensity and disappear after a certain time. The former is due to the thermal heating of the lattice and the loss of long-range order associated with crystalline phase when melting occurs. The disappearance of Bragg peaks indicates the complete loss of the crystalline structure, and what remains is the short-range correlations of strongly coupled ions in liquid phase warm dense matter. Another feature can be seen is the appearance of the Debye-Scherrer ring, an indication of the highly disordered liquid state existing in the system. In summary, the results of these measurements can be used to benchmark the theories of lattice dynamics, as well as electron-ion coupling and ion-ion correlation in non-equilibrium WDM.